1. Field of the Invention
The present invention relates to an optical glass having the optical constants of a refractive index nd of 1.70 or greater and an Abbé number νd of 50 or greater, a preform for precision press molding comprised of this glass, an optical element comprised of this glass, and methods of manufacturing the same.
2. Discussion of the Background
With the advent of digital cameras and cellular telephones equipped with cameras, the degree of integration and the high degree of functionality of devices employing optical systems have advanced rapidly. At the same time, the demand for high precision, lightweight, compact optical systems is increasing.
In recent years, to meet the above need, optical designs employing a spherical lenses have been increasingly coming into the mainstream. Thus, to stably supply large numbers of a spherical lenses employing highly functional glass at low cost, precision press molding (also known as a mold pressing technique), in which an optical functional surface is directly formed by press molding without grinding or polishing steps, has attracted attention. The demand for optical glass having low temperature softening properties suited to precision press molding is increasing each year. Among such optical glasses, there are high refractive index, low dispersion glasses. An example of such a glass is described in Japanese Unexamined Patent Publication (KOKAI) No. 2002-249337 or English language family member U.S. Patent Application No. 2003-125186 AA, which are expressly incorporated herein by reference in their entirety.
To take full advantage of the above precision press molding technique, a glass material known as a preform is desirably directly formed from glass melt. This method is known as the preform hot molding method. A glass melt is caused to flow out, glass melt gobs of a weight corresponding to single preforms are separated one after another, and the glass melt gobs obtained are cooled to form preforms having smooth surfaces. Accordingly, in contrast to the method of forming large glass blocks from glass melt and cutting, grinding, and polishing the glass blocks, this method affords excellent characteristics in the form of a higher glass use rate, the fact that no glass shavings are produced during processing, and zero processing time and cost.
However, in the hot molding method, a glass melt gob of a weight corresponding to a single preform is accurately separated and a preform molded in such a manner that devitrification and defects such as striae do not result. Accordingly, a glass having good glass stability in the high temperature range of hot molding is required.
When the refractive index nd is raised while maintaining an Abbé number νd of a prescribed level or above, the tendency of the glass to crystallize intensifies, thereby compromising vitrification. Since a low temperature softening property is further imparted to the glass employed in precision press molding, there tends to be a drop in glass stability. Accordingly, it is difficult to realize glass stability at a level permitting the hot molding of preforms while maintaining an Abbé number νd of 50 or greater, preferably 52 or greater, a refractive index nd of 1.70 or greater, and a low temperature softening property suited to precision press molding.
The present invention, devised to solve the above-described problems, has for its object to provide an optical glass exhibiting good glass stability while having a refractive index nd of 1.70 or greater, an Abbé number of 50 or greater, and a low temperature softening property; a preform for precision press molding comprised of this glass; a method for manufacturing this preform; an optical element comprised of such glass; and a method for manufacturing such optical elements.